1. Field of the Invention
The present invention relates to a fixing unit used in an image forming apparatus such as an electrophotographic system.
2. Description of the Prior Art
In a copying machine of an electrophotographic system, an original document image is exposed to light and the reflected light is irradiated to a photosensitive drum to form an electrostatic latent image thereon, and toner is supplied to the electrostatic latent image, whereby a toner image is formed. The toner image on the photosensitive drum is transferred onto a recording sheet by a transfer unit and is fixed onto the recording sheet by the fixing unit.
The fixing of the fixing unit is performed by heating toner and pressing the thus melted toner against the recording sheet. The fixing unit has, for example, a heat roller and a pressure roller which presses against the heat roller. A heat lamp is inserted into the heat roller.
In the past, the rotation speeds of the heat roller and the pressure roller in the fixing unit have been almost constant between the time when the recording sheet is supplied and the time when it is discharged. Because of this, when a large sized recording sheet is used, or copying is performed at a high speed, the surface temperature of the heat roller is transmitted to the recording sheet at a high rate. Thus, the surface temperature of the heat roller is partly or temporarily decreased. Such a decrease in the surface temperature causes the difference in temperature between the leading edge and the trailing edge of the recording sheet to be supplied. In the conventional fixing unit, when a plurality of copies are made, it has been difficult to secure uniformity of the fixing at the initiation of copying and the finishing of copying. In addition, the difference in the fixing property is caused between the leading edge and the trailing edge of the same recording sheet.
In order to avoid such a decrease in the surface temperature of the heat roller, the heat roller can be made larger so as to increase the thermal capacity of the heat roller itself. However, this causes the copying machine to be enlarged, leading in an increase in cost.